1. Field of the Invention
The present invention relates to manufacturing of a charge injection type electroluminescent device using an organic thin-film material.
2. Description of the Related Art
There is an electroluminescent (EL) device as a light-emitting device recently expected to be applied to and partially embodied into a light-emitting unit of a display or a surface light source.
FIG. 12 shows a structure of a prior-art EL device using an organic thin-film material (hereinafter referred to as an organic EL device). The organic EL device 1 comprises a transparent substrate 2, a transparent electrode 3 formed on an upper surface of the transparent substrate 2 and made, e.g., of indium tin oxide (ITO), a thin-film luminescent layer 4 formed on an upper surface of the transparent electrode 3, a thin-film electron transport layer 5 formed on an upper surface of the luminescent layer 4, and a back electrode 6 formed on an upper surface of the electron transport layer 5 and serving as an electron injection electrode.
The luminescent layer 4 serves as a hole transport layer. Fluorescent pigment is previously dispersed in the luminescent layer 4, so that the luminescent layer 4 operates. Electrons injected into the luminescent layer 4 and holes injected into the luminescent layer 4 by means of a field applied between the back electrode 6 and the transparent electrode 3 are recombined primarily in the luminescent layer 4. This generates excitons. A movement of the excitons causes the fluorescent pigment to emit a visible light in accordance with the kind of the fluorescent pigment.
A layer including a region in which electrons and holes such as the luminescent layer 4 are recombined are called a recombination region layer, hereinafter.
In manufacturing the organic EL device described above, particularly in forming the luminescent layer 4, i.e., in making the fluorescent pigment disperse into a dispersion medium layer, a material for the dispersion medium layer and the pigment to constitute dispersoid are dissolved in a common solvent. The resulting solution is applied by dip coating or spin coating to the transparent electrode 3. The applied solution is dried to provide the dispersion medium layer (i.e. a hole transport layer) in which the dispersoid pigment (i.e. fluorescent pigment) is dispersed.
This organic EL device has the following advantages that:
(1) A luminescent wavelength is optionally set since optional fluorescent pigment can be dispersed; PA1 (2) Since a common solvent is used, a material for the luminescent layer can be used, even if it has a low layer forming capability; and PA1 (3) When a low-crystallizable dispersion medium layer material such as a polymer is employed the above manufacturing process prevents a failure due to the crystallization of a luminescent material, so that a reduction in the service life of the organic EL device due to the failure can be solved.
In addition, since the organic EL device can theoretically more easily emit a blue light than an inorganic EL device, it is expected to be applied to discrete red, green and blue (RGB) luminescent devices.
In the wet process employing the dip coating or spin coating for forming the luminescent layer of the organic EL device of FIG. 12, it is important to select the common solvent in forming the luminescent layer. On the other hand, the thicknesses of the charge transport layers of the organic EL device comprising the electron transport layer, the hole transport layer and if necessary a bipolar transport layer formed between the electron and hole transport layers must be hundreds to thousands angstroms. Thus, if a common solvent is employed in order to meet a layer forming requirement in the wet process, a requirement for forming an optimum layer cannot always be established for the dispersion medium layer material. Thus, it is difficult to form an sufficient dispersion medium layer.
It has been difficult to produce the organic EL device in the form of discrete RGB luminescent devices. This is considered primarily because it is difficult to produce organic thin films constituting RGB pixels in the form of a matrix pixel pattern on the same substrate by, for example, the lithography or screen printing.
Thus, a process for manufacturing an organic electroluminescent device able to provide luminescent colors in accordance with kinds of fluorescent pigments must meet the requirements that the fluorescent pigments provide desired luminescent colors, and the dispersion medium allows the fluorescent pigments to uniformly disperse, and the common solvent easily dissolves the fluorescent pigments and the dispersion medium, and is volatilized without deteriorating the nature of the fluorescent pigments.
In addition, this process must comprise, for each color, the many difficult sequential steps of applying a solution of a kind of fluorescent pigment, a dispersion medium and a common solvent, forming a thin film by the spin coating or the like, drying the thin film, and etching the dried thin film with the masking so that necessary portions remain to form a desired pixel pattern.
This process of the manufacturing the EL device includes, for example, forming a first luminescent layer including a fluorescent pigment which is capable of emitting a first color, etching the luminescent layer into a predetermined pattern, and forming a second luminescent layer including a fluorescent pigment which is capable of emitting a second color different from the first color and etching the second luminescent layer into a predetermined pattern adjacent to the pattern of the first luminescent layer. However, it is difficult to etch the first and second luminescent layers into accurate patterns, and consequently, there is an interval or an overlap between the patterns of the first and second luminescent layers.
Since a top electrode or the carrier transport layer is arranged on the first and second luminescent layers, the electrode or the layer is discontinued easily and their surface may be uneven. Hence, the electrical and optical characteristics of the multicolored EL device deteriorate.